1. Field of the Invention
The present invention relates to the interworkings of a combined Circuit-Switched call and IMS services (CSI) including a Circuit-Switched (CS) and an Internet Protocol Multimedia Subsystem (IMS) session combined together, and in particular, to a method and system for communication between a user equipment (CSI UE) which can simultaneously support the CS call and IMS session, and a User Equipment (IMS UE) which cannot support the CSI and can support only the IMS session.
2. Description of the Related Art
A Circuit-Switched (CS) scheme is used for typical voice service. The CS scheme forms a CS-based fixed communication path between two users. In comparison, an Internet Protocol Multimedia Subsystem (IMS) uses a Packet-Switched-based (PS-based) variable communication path based on an Internet Protocol (IP), thus increasing transmission efficiency and ensuring stability. Such an IMS can support a connection to a session in which a plurality of users participate, as well as one-to-one communication. According to the CS scheme, a message is transmitted through a CS call. In comparison, according to the IMS scheme, a message is transmitted through an IMS session.
Service enabling a UE to simultaneously use the CS call and IMS session is called “CSI”. The CSI uses a CS call for voice transmission, that is, for voice communication, and simultaneously uses an IMS session for an enhanced Packet-Switched (PS) service including a messaging or file transmission. A UE supporting the CSI is called a “CSI UE”. The CSI UE can use a CS call for realtime voice communication while using an IMS session for non-realtime PS services such as messaging. While the CSI takes voice communication into consideration as a service using a CS call, the CSI enables a CS call to be used for image communication as well as voice communication when a CS network supports the image communication.
An IMS network includes a Message Gateway Control Function (MGCF) which is an IMS entity taking charge of interworking between a CS-call-related message (CS message) and an IMS-session-related message (IMS message). The MGCF provides a function of converting a Session Initiation Protocol (SIP) header and Session Description Protocol (SDP) parameters, which are basically included in a message, into an Integrated Services digital Network (ISDN) User Part (ISUP)/Bearer Independent Call Control (BICC) message, and a function of converting an ISUP/BICC into a SIP header and SDP parameters. According to the CSI, a CSI UE attempts capability exchange with a peer UE by transmitting a CS-call setup message including the capability information of the CSI UE. In this case, the peer UE may be another CSI UE, an IMS-only UE which cannot support the CSI and can support only the IMS session, a Voice-Call-Continuity (VCC) UE which supports VCC service, or a UE which can support both the CSI and VCC service. Through the capability exchange, it is possible to determine if each UE can use the CS service for a voice call, provide the IMS service for data transmission, or provide both the CS service and IMS service.
When a call setup is generated between a CSI-capable UE (i.e., CSI UE) and a CSI-incapable UE (i.e., IMS terminal), the CSI UE transmits a CS-call setup message including its own capability information to the IMS UE. However, since a conventional MGCF has no method of converting the capability information transmitted by the CSI UE into a SIP message, the MGCF cannot transfer the capability information transmitted by the CSI UE to the IMS UE. As a result, it is impossible to perform capability exchange through a CS message between a CSI UE and an IMS UE, resulting in no direct IMS connection between the two UEs requesting an IMS connection setup although both the UEs can support a connection to the IMS.